An optical waveguide with its core being formed of a medium with a high refractive index such as silicon barely generates an optical loss even in the case of a sudden bend, thereby making it suitable for integration of optical circuits. One of optical waveguides attracting attention is an optical waveguide using a Silicon-on-Insulator (SOI) substrate.
The SOI substrate is composed of a silicon base substrate, a Buried Oxide (BOX) layer thereon, and a silicon layer (SOI layer) on the BOX layer. With the SOI substrate, the SOI layer is processed to form a core with a high refractive index, and the BOX layer can be used as lower cladding with a low refractive index, thereby facilitating manufacture of a silicon thin wire waveguide (channel waveguide) and a ridge waveguide (rib waveguide). An oxide film to be top cladding is deposited as necessary.
A core of the silicon thin wire waveguide for an optical circuit generally has a rectangular or square cross-section, and in the case of a single-mode, a core with an approximately 400 nm width and 200 nm height, for example, is often used. Moreover, a core with a structure including slabs (plate structure) with a thickness of 50 to 100 nm on both sides of the silicon thin wire waveguide is often used for a core of a silicon ridge waveguide.
As in the above explanation, since the silicon thin wire waveguide and ridge waveguide for optical circuits have a fine core, a mode field of light propagating therethrough is also fine with its width and height about less than or equal to 1 μm. Although a small core size and mold field size are advantageous to integration of optical circuits, they pose a problem in optical coupling with an external optical system.
As for a general single-mode optical fiber for transmitting an optical signal, a diameter of a core is about 8 μm, and a diameter of the mode field size at a wavelength 1.55 μm is about 10 μm, which are larger than those of the thin wire waveguide.
A cheap optical coupling method is to establish a connection by butting a cutting endface of an optical fiber to an endface of a waveguide, which is an input and output end of an optical circuit. However, an optical conversion element for converting a spot size must be created at the input and output end of the optical fiber in order to achieve high optical coupling efficiency by this butt coupling.
Note that the mode field size is a size of a field (usually an electric field) in an eigenmode of a waveguide. In this specification, the mode field size indicates a field size in a fundamental mode except as otherwise specified. Further, the spot size is the field size of light immediately after being emitted from a waveguide. In the case of a multi-mode waveguide, light in several eigenmodes may be mixed.
However, this specification assumes only the fundamental mode as described above when referring to the mode field, thus the mode field size and the spot size shall be used in the same meaning.
Non-patent literature 1 discloses a spot size converter (optical conversion element) (hereinafter referred to as a first technique). The spot size converter has a dual core structure including a first core formed of a silicon thin wire and a second core with a larger cross-sectional area than the first core and disposed to cover the first core. Further, a first core has a tapered structure with its width decreasing gradually toward a side to be connected to an optical fiber.
An operation of the spot size converter according to the first technique is explained below. Most of the mode field of light entered to the first core from a thin wire waveguide side is distributed within the first core at a wide width part. When guided light advances and the width of the waveguide is reduced sufficiently lower than a half waveguide, the mode field overflows outside the first core and is filled within the second core. Finally, the size of the mode field expands to the size of the second core, thereby facilitating optical coupling with an optical fiber.
The configuration of spot size conversion that expands the mode field by gradually narrowing the core size to less than or equal to the half wavelength is referred to as a reverse tapered type as the direction to which the core size expands is opposite to the direction to which the mode field size expands.
It is necessary to conform the size and the shape of the mode field of the guided light in order to improve the optical coupling efficiency of two waveguides, however it is also necessary to conform effective refractive indices of the guided light at the same time. The effective refractive index is related to a strength ratio of the electric field and magnetic field, and has a value between the refractive index of the material forming the core and the refractive index of the material forming the cladding, depending on the structure of the waveguide.
Although the mode field usually indicates distribution of an electric field, high optical coupling efficiency cannot be achieved unless distribution of magnetic field is conformed at the same time. Conforming the refractive indices together with the mode fields is equivalent to conforming the distribution of the electric field and distribution of the magnetic field of the guided light. When the effective refractive indices are not conformed, light is reflected at an interface between the connected two waveguides, thereby reducing the optical coupling efficiency.
The spot size converter according to the first technique can largely change the refractive indices of the material forming the first core and the material forming the second core. Even when the material forming the first core is material with a high refractive index, with a small cross-sectional area of the core, the effective refractive index of the guided light will be close to the refractive index of the material forming the second core. As the cross-sectional area of the second core is large, when the refractive index of the material forming the second core is made to be the same degree as the refractive index of the material forming the optical fiber, the effective refractive index of the guided light in the second core can be made close to the effective refractive index of the guided light in the optical fiber.
The spot size converter with the tapered structure realizes high conversion efficiency by adiabatically changing the mode field. An issue in the spot size converter according to the first technique is that the larger a difference in the sizes of the first core and the second core, the more difficult an adiabatic change in the mode field would be. Theoretically, when the size of the first core is made close to zero, it is possible to expand the size of the mode field to infinite.
However, in practice, the greater the ratio of the mode field expanded outside the first core, the larger the ratio of the change in the mode field size to the change in the size of the first core, thereby making a gradual change in the mode field size difficult.
In order to still achieve a gradual change in the mode field size, the change in the reverse taper width of the first core needs to be further gradual as long as a limit of processing accuracy is not including resolution accuracy of lithography and surface roughness by etching. Consequently, the size of the mode field that can adiabatically change is limited.
In the spot size converter disclosed in the first technique, such a reason limits a mode diameter (diameter) of the optical fiber that is optically coupled by butt coupling of the silicon thin wire waveguide to 4.3 82 m. Since the mode diameter (diameter) of the single-mode optical fiber generally used in the optical communication is about 10 μm, the spot size converter disclosed in the first technique cannot be used.
There are other structures suggested for the optical conversion element that can expand the spot size of the silicon waveguide. They operate by a different mechanism from the spot size converter disclosed in the first technique.
Patent literature 1 (hereinafter referred to as a second technique) discloses a spot size converter having a structure that covers a thin wire core by material with an almost same refractive index as that of material forming the thin wire core, which is a first core, in a way that the material is gradually thicker toward an endface on an optical fiber side. The covering material and the entire first core compose a second core. Since the covering material and the material forming the first core is almost the same, the structure can be considered as being formed by simply expanding a cross-sectional area of the first core.
In the spot size converter according to the second technique, as a cross-sectional size of a core of a waveguide is greater than a half wavelength of guided light, a mode field of the guided light expands along with an expansion in the cross-section of the core. Since the change in the cross-sectional size of the core and the change in the mode field size are the same, this is a forward tapered spot size converter.
In the forward tapered converter, the larger the mode field size increased by the expansion, the closer the mode field and the core size would be. As a result, the change in the mode field size will be about the same degree as the change in the core size, thereby facilitating control of the cross-sectional size of the core that gradually changes the mode field size.
Therefore, even when there is a large difference in the cross-sectional size of the core before and after the conversion, limitation in manufacture as in the first technique would not be generated. Hence, the mode field size can be conformed to a general optical fiber with a mode diameter (diameter) of 10 μm.
Although the spot size converter by the second technique includes a gradual vertical taper, a formation technique of such vertical taper cannot be used in some cases. Then, there is a structure suggested that is capable of expanding/reducing the mode field size in the thickness direction only by creating a taper in the vertical direction without forming the vertical taper.
Non-patent literature 2 (hereinafter referred to as a third technique) discloses a spot size converter that expands a core size without hardly changing a refractive index of material forming a core in a similar manner as the spot size converter according to the second technique.
The spot size converter according to the third technique adds a twist to the method of expanding the cross-sectional size of the core from a thin wire waveguide side. The structure formed by vertically stacking tapers with only its width gradually changing can change not only the horizontal size of the mode field but also the vertical size thereof.
In a part where the upper tapered structure is sufficiently thin, most mode field of the guided light is distributed inside the lower tapered structure, thus it is possible to avoid a sudden change in the mode field size even with a sudden step in the thickness direction. As a sudden change in the height of the structure can he created, the formation technique of the tapered structure in the thickness direction is not necessary.
However, the second and third techniques have a common issue. The issue is that the effective refractive index of the guided light cannot be largely changed as the refractive indices of the material forming the first and second cores are almost the same. For example, when the first core is silicon, the second core is also silicon.
In the case of expansion in the core size with such refractive index of silicon, the effective refractive index of the guided light will he about 3.5 of the refractive index of the silicon. Meanwhile, the effective refractive index of guided light in general optical fibers for communication is about 1.5, and simply connecting the spot size converter to this optical fiber will reduce the optical coupling efficiency clue to interface reflection.
Anti-reflective coating on an endface of the waveguide of the optical circuit suppresses the reflection but it is not suitable for high-volume manufacturing. This is because each one of optical circuit chips must be applied with the anti-reflective coating, thereby not reducing the production cost by high-volume manufacturing. The same problem arises when the anti-reflective coating is applied on the side of optical fibers instead of the optical circuit chips.
As stated above, the spot size converters according to the first to third techniques have issues including the limitation of the maximum expandable size of the mode field and necessity of anti-refractive coating. However, the spot size converters can at least establish optical coupling between the waveguide of the optical circuit and the optical fiber.